Many pourable food products, such as for example fruit juice and UHT milk, are sold in packages made of sterilized packaging material. An exemplary type of package is the one known as Tetra Evero Aseptic (registered trademark), which is a carton bottle manufactured in a highly automated filling machine. A web of laminated packaging material is cut in blanks or sheets. Each blank or sheet is formed as a sleeve and overlapping longitudinal edges of the sleeve are sealed together. Said sleeve is subsequently closed in one end in that a top of thermoplastic material is injection moulded directly on the sleeve end portion. The thermoplastic top comprises a pouring spout which is closed by for instance a screw cap. The pourable food product is filled into the package through the open sleeve end, which after filling will be sealed and folded into a flat bottom.
The packaging material has a multilayer structure substantially comprising a base layer of fibrous material, e.g. paper or carton, and a number of layers of heat-sealable polymeric material, e.g. polyethylene films, covering both sides of the base layer. The packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil, which is positioned in between the base layer and the heat-sealable polymeric material forming the inner face of the package eventually contacting the food product. Between the aluminium foil and the base layer there is formed an adhesion layer.
The longitudinal seal of the sleeve is made by a sealing device. Sealing devices have been used in the liquid food packaging industry for many years. Different types exist and they can be electrically heated, use ultrasonic vibrations or, where possible, utilize an electrically conductive material such as the aluminium foil layer to generate heat suitable for sealing. The latter is a so-called induction heat-sealing process, in which, eddy currents, i.e. loss currents, are induced in the aluminium layer, resulting in a localized heating and melting of the heat-sealable polymeric material layers.
More specifically, in induction heat sealing, the sealing device substantially comprises an inductor powered by a high-frequency current generator. The inductor is a coil made of electrically conductive material and the generated electromagnetic field interacts with the aluminium layer in the packaging material to induce eddy currents in it and heat it to the necessary sealing temperature. The coil is typically made of e.g. a conductive and/or metallic material such as e.g. copper.
When the heat-sealable polymeric layers on the inside and/or outside of the packaging material are melted, a sealing can be made by simultaneously pressing two heated layers together. The pressing action is accomplished by the induction sealing device and a counter-sealing element such as an anvil, that may be fitted in a sleeve-forming mandrel.
Another type of package for pourable food products is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated web packaging material.
Similarly, the packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene films, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this latter sort are produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging unit, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is fed continuously in a first vertical direction, is filled with the sterilized or sterile-processed food product, and is gripped at equally spaced cross sections by two pairs of jaws. More specifically, the two pairs of jaws act cyclically and successively on the tube, and heat seal the packaging material of the tube to form a continuous strip of pillow packs connected to one another by respective transverse sealing bands, i.e. extending in a second direction perpendicular to said first direction.
The pillow packs are separated by cutting the relative transverse sealing bands, and are then fed to a final folding station where they are folded mechanically into the finished parallelepiped shape.
The tube portion gripped between each pair of jaws is heat sealed by heating means fitted to one of the jaws, known as the sealing jaw, and which locally melt the two layers of heat-seal plastic material gripped between the jaws.
The transversal sealing is performed by induction sealing, and the heating means substantially comprise an inductor powered by a high-frequency current generator and substantially comprising one or more inductor bars made of electrically conductive material, extending parallel to the second direction, and which interact with the tube material to induce a loss current in it and heat it to the necessary sealing temperature.
The other jaw, known as the counter-jaw or anvil, on the other hand, is fitted with pressure pads made of elastomeric material, and which cooperate with the inductor bars to heat seal the tube along a relative transverse sealing band.
Sealing jaw sealing devices comprising inductors of the above type are known.
More specifically, exemplary known sealing devices substantially comprise a supporting body connected integrally to the sealing jaw and defining two front seats for housing respective inductor bars; and an insert made of magnetic flux-concentrating material—in particular, a composite material comprising ferrite—and housed inside the supporting body, close to the inductor bars.
More specifically, the inductor bars are simple single or double coils for creating a magnetic field and the inserts are made of magnetic flux-concentrating material. Such known sealing devices are e.g. known from the patent documents EP 1 270 182 and EP 2 008 795.
A common problem with a single loop transversal sealing inductor is the need of different magnetic fields or different heating powers along the length of the inductor. Extra heating power is needed in the area corresponding to the corners of the packages as well as in the cross area where packing material overlap, because these areas are more difficult to seal. Currently discreet extra parts with higher magnetic properties are used for this purpose. This results in complexity and cost and also in making the manufacturing process more difficult. Also, in order to keep all the components together, an external shell of plastic material has to be used, that further increases the complexity and vulnerability to weaknesses in the plastic shell.
Since all induction sealing technology relies on the use of the aluminium layer of the packaging material, the induced heat is not always generated in the optimal way and/or not in the optimum location. Hence, there is always room for improvements with regard to the inductors used for induction sealing.